Many contemporary engine controls have integral misfire detection systems. With ever-increasingly more stringent emissions standards, the assurance of accurate and complete misfire detection under all engine and vehicular operating conditions is becoming mandatory.
Commonly, system designers rely on measurement of crankshaft engine angular velocity, and sometimes crankshaft or other forms of, engine acceleration, both dependent largely on engine torque produced during a combustion process to determine misfiring of a particular engine cylinder. Given the velocity or acceleration information, misfires are predicted by various signature analysis, and/or spectral analysis, methods.
As a practical matter the engine angular velocity and acceleration behavior is also affected by powertrain related behaviors other than firing torque. These other behaviors can significantly reduce fidelity or signal-noise ratio of the primarily firing torque related velocity or acceleration signal under analysis. Furthermore, under some engine operating conditions, the noise exceeds the primarily engine torque related velocity or acceleration signal under analysis. Moreover, the noise related behavior is not limited to engine operation only causes, but include behaviors related to the complete driveline. Some noise related behaviors that are detrimental include driveline resonance effects, or vibrations, excited at least partially by cylinder misfiring, torque converter lockup, low speed lugging behavior characteristic of a manual transmission, a change in transmission gears and rough road conditions. Each of these, and other sources of stimulus, excite the driveline to perturbate, or transiently oscillate, at its resonant frequency.
When the above-mentioned driveline behaviors manifest themselves and the driveline oscillates a significant measure of what amounts to noise, relative to the misfire induced behavior, is introduced into the velocity or acceleration measurement. This noise can largely swamp out any signatory behavior of a misfire event--particularly with a noncompliant coupling between the engine and the transmission.
FIG. 1 shows a 1st portion 101 of a waveform indicative of acceleration of an engine's crankshaft due to a properly firing cylinder, firing in a sequence of several cylinders, and a 2nd portion 103 waveform indicative of acceleration of an engine's crankshaft due to a misfiring cylinder later in the sequence of firing cylinders. Of note, at reference number 103 the engine's crankshaft grossly decels because proper firing did not occur and therefore the cylinder did not add the expected torque to the crankshaft as it did at reference number 101. Note that his is a theoretical representation of the acceleration effect. If the actual behavior produced by the engine crankshaft is that shown in FIG. 1 then a comparison process can monitor the behavior at a predetermined threshold 105 and indicate a misfiring condition if the waveform goes below the threshold.
FIG. 2 illustrates a behavior of an actual acceleration signal 201 derived from a running engine over about 150 cylinder combustion cycles. This acceleration signal 201 includes a repetitively induced misfire by periodically removing a spark signal from one cylinder. So, in a real-world application the signal derived from a running engine is effected by other than combustion related torque as earlier mentioned. For reference purposes, the tick lines 203 on the horizontal axis demarcate the deliberately induced occurrences of misfire. The waveform 201 is derived using an acceleration sensing device coupled to the engine's crankshaft. Because of crankshaft torsional vibrations, inertial torque due to reciprocating masses, and other mechanically induced vibrations on the engine's crankshaft, the waveform shown in FIG. 2 has poor fidelity and therefore the acceleration signal related to the misfire cannot be clearly seen and thereby detected as discussed in FIG. 1. Additionally, the above-mentioned driveline resonance effects are also represented in the subject waveform in FIG. 2 and significantly reduce the fidelity of the signal making detection by a simple threshold detection scheme hopeless.
One prior art scheme averages the engine acceleration waveform to improve the fidelity by eliminating cylinder-to-cylinder variability at least partially influenced by engine related vibrations. However, these fixed averaging schemes perform inadequately over all engine operating conditions--particularly at high engine speeds and light engine loads.
Another prior art scheme applies a chassis mounted accelerometer to predict driveline vibrations characteristic of a subset of the above-mentioned behaviors. Accordingly, if one of these behaviors is sensed the misfire detection scheme is disabled. This prevents false detection of what appears to emulate a misfire but is in fact a driveline perturbation caused by, for instance, a rough road condition. Of course this scheme will also fail to detect misfires occurring during this disabling process. Abdication to predict meaningful misfires aside, this scheme is complex, unreliable, difficult to manufacture, and very costly because it adds another sensor and its associated supporting wiring, electrical interface, and signal conditioning circuitry.
Another scheme employs a fixed rank median filter to determine gross, or average, acceleration of an engine which is then used to filter-out certain transient effects. This scheme lacks the ability to adapt to a wide variety of driveline vibrations caused by various sources of stimulation, not insignificantly misfire induced driveline vibrations, as described above. As in the former scheme, this scheme includes recognition of poor acceleration signal fidelity and disables the misfire detection mechanism when the fidelity is poor. Accordingly, the known prior art only detects a subset of misfiring conditions. This is unacceptable with the strict legislation proposed.
What is needed is an improved approach for misfire detection particularly one that is insensitive to averse powertrain operating effects. In particular, an improved system needs to account for crankshaft torsional vibrations, inertial torque due to reciprocating masses, other mechanically induced vibrations on the engine's crankshaft, and driveline perturbations over a wide range of engine operating conditions.